Sleep often affects healthy individuals differently from those with heart and/or respiratory ailments. In healthy individuals, sleep generally exerts a salutary and restorative influence. In patients with respiratory and/or heart ailments, by contrast, sleep may bring on breathing disorders, myocardial ischemia, arrhythmias, and even death. Two main factors thought to be responsible for such reactions are sleep-state dependent changes in autonomic activity and depression of respiratory control mechanisms.
Sleep states include rapid eye movement (REM) state and non-REM, or slow wave sleep (SWS) state. The brain, in serving its needs for periodic reexcitation during REM sleep state and dreaming, may impose significant demands on the heart by inducing bursts of sympathetic activity which may, in susceptible individuals, compromise coronary blood flow and trigger life-threatening arrhythmias. During SWS state, hypotension may lead to under-perfusion of the heart and brain as a result of a lowered pressure gradient through stenosed blood vessels. Also, impairment of breathing ventilation during sleep by obstructive sleep apnea may cause reductions in arterial oxygen saturation. These conditions may be worsened by cardiac medications which cross the brain-blood barrier, altering sleep structure and possibly leading to nightmares with severe cardiac autonomic discharge.
Patients with coronary artery disease face nocturnal health risks. Approximately 25% of cases of first manifestations of underlying coronary artery disease include catastrophic events such as myocardial infarction and sudden death due to respiratory or cardiac distress during sleep. In a recent study, it was observed that approximately 20% of myocardial infarctions and 15% of sudden deaths occur during the period between midnight and 6:00 A.M. Such a non-uniform distribution of myocardial infarctions and sudden deaths suggests that sleep-state dependent fluctuations in autonomic nervous system activity may have precipitated a significant number of the events.
From the data, an estimated 300,000 nocturnal myocardial infarctions and 37,500 nocturnal sudden deaths occur yearly in the U.S. The latter figure is approximately equal to 88% of the number of deaths due to automobile accidents and is approximately 50% greater than the number of deaths due to HIV infection. Sudden death during sleep often victimizes infants and adolescents, and adults with ischemic heart disease, who have a median age of 59.
Different pathophysiologic mechanisms are responsible for nocturnal death in different age groups and among different patient groups. Disturbed respiration during sleep is particularly dangerous for patients with heart-related and certain other ailments. Respiratory apnea, for example, which afflicts approximately 1.5 million Americans, may bring on hypertension, myocardial infarction and sudden death, particularly in individuals with ischemic heart disease. Patients with heart failure, those with warning signs of Sudden Infant Death Syndrome (SIDS), pause dependent long QT syndrome or Sudden Unexplained Nocturnal Death Syndrome (SUNDS) (which primarily affects young Southeast Asian men) also are at particularly high risk for dangerous nocturnal cardiorespiratory events. Mobile coronary care unit reports indicate a higher nighttime incidence of atrial fibrillation and arrhythmia which is responsible for significant morbidity and mortality.
Because of the known risks associated with sleep in patients with cardiac ailments and other health problems, monitoring sleep state and respiratory pattern in conjunction with synchronized cardiac monitoring, would aid in the treatment of such patients. Particularly, such monitoring would aid in the diagnosis of ailments, would offer specific information on particular patients' sleep state-dependent cardiac response, and would help in the prevention of dangerous nocturnal cardiac events. Additionally, because certain cardiac medicines, particularly those which cross the brain-blood barrier, affect heart function in often unknown and sometimes dangerous ways during sleep, monitoring sleep state in conjunction with cardiac response would aid in the selection of appropriate medicines and dosages.
Despite the known risks associated with sleep in certain individuals, however, sleep state monitoring has not been an integral component of cardiac patient care, largely because hospital-based or ambulatory monitoring sleep studies are complex and expensive and also because simple, yet accurate, equipment for monitoring sleep and heart function in the familiar home environment is not available.
Hospital-based systems are the most accurate for monitoring sleep state. One such system provides detailed information about and individuals brainwave activity through electroencepholographic (EEG) signals, eyelid movement through electrooculographic (EOG) signals, and muscle tone through electromyographic (EMG) signals. An electrocardiogram also provides information on heart function through electrocardiographic (ECG) signals. Typically, arterial blood pressure, respiration, oxygen saturation, and body movements also are recorded with hospital-based systems. The recorded events are temporally synchronized for later analysis and study.
Notwithstanding its thoroughness, there exist a number of drawbacks with the hospital-based system. The procedure is disruptive to a patient's sleep as it requires the patient to wear multiple scalp electrodes, a blood pressure cuff and respiratory recording devices. It also is disruptive to the patient's sleep because it is conducted in an unfamiliar environment. As a result, a highly disruptive "first night" effect occurs, which includes sleep that is lighter and more fragmented than usual and which may last up to 12 consecutive days. The results obtained, therefore, may not accurately reflect what will occur when the patient is released from the hospital. Another significant limitation to the hospital-based system is the cost of use as it requires a trained technician and sleep scorer to operate. Moreover, it includes sophisticated and expensive equipment and it requires a hospital stay for the patient.
The field approach equivalent, which best approaches the accuracy and thoroughness of the hospital-based system, is the system used in ambulatory polysomnography (PSG). The ambulatory PSG system is a portable system available for home-based use that includes surface electrodes connected to the scalp, eye and body. A portable recorder receives and records EEG, EOG, ECG and EMG signals. Portable equipment also exists for measuring respiration, oxygen saturation, temperature and body movement. All signals are temporally synchronous and, like the hospital-based system, are recorded first and then analyzed.
While the ambulatory PSG system has the advantage over the hospital-based system that it may be used in the familiar home environment and it appears to reveal a reduction in first night effects, it also suffers from a number of drawbacks. For example, the system may negatively affect sleep quality due to the bulk and weight of the recorders and the need for multiple scalp and body electrodes. Additionally, a technician is required to apply the electrodes and the data produced is complex, thus requiring analysis of such data by trained personnel, adding significantly to the cost of its use.
U.S. Pat. No. 5,187,657 to Forbes, titled Cardiac Analyzer with REM Sleep Detection, discloses a home-based system for monitoring sleep state and cardiac function, much like the PSG system. The system disclosed uses sleep state sensors to detect eyelid movement, and muscle tone or brain wave activity, from which sleep state is inferred. When REM state is detected, then recording of ECG and other signals is initiated. Cardiac events attributable to SWS state phenomena or to disturbed respiration are thus not monitored.
In each of the hospital-based and ambulatory PSG systems, as well as in the system disclosed in the Forbes patent, sleep state is determined as a function of eyelid and/or head movement but not as a function of heart function. The accuracy of sleep state data therefore is limited. Additionally, the hardware required for measuring respiratory pattern is separate from the other equipment and, therefore, adds to the expense and may further disrupt sleep quality.
A more simple sleep state monitoring device available for home use is described in U.S. Pat. No. 4,836,219 to Hobson, titled Electronic Sleep Monitor Headgear, and owned by the President and Fellows of Harvard College, which patent is incorporated by reference herein in its entirety. The Hobson patent is licensed to Healthdyne which sells the device under the name Nightcap. The Nightcap device includes a piezoelectric sensor that attaches to the eyelid of a patient and a tilt sensor connected to a headband worn by the patient. The eyelid sensor and tilt sensor provide electronic signals respectively representative of eyelid movements and head movements to recording circuitry through electric wires. Sleep state is determined based on the eyelid and head movement information.
While the Nightcap is available for home use, it requires the patient to wear a headband which may negatively affect the sleep quality of the patient. Also, the Nightcap device is limited to monitoring sleep state; it does not also monitor heart function, for example.
None of the prior art systems discussed above provide a simple, self-contained, home-based system that assesses sleep-related cardiorespiratory risk.
Accordingly, one object of the present invention is to provide a simple system and method for assessing cardiorespiratory risk that is available for home use and which minimally affects sleep quality.